Estimating Leachate Production From Closed Hazardous Waste Landfills


                    United States
                    Environmental Protection
                    Agency
Hazardous Waste Engineering
Research Laboratory
Cincinnati OH 45268
                    Research and Development
EPA/600/S2-86/057 Sept. 1986
&EPA          Project  Summary

                    Estimating   Leachate
                    Production  from  Closed
                    Hazardous  Waste  Landfills
                    R. R. Kirkham, S. W. Tyler, and G. W. Gee
                      Hazardous wastes disposed of in
                    landfills may continue to drain for sev-
                    eral years after site closure. Leachate
                    sources include waste fluids as well as
                    precipitation trapped in the landfill dur-
                    ing construction and operation. Waste
                    fluids may be released via barrel degra-
                    dation and subsidence and/or compres-
                    sion of waste materials. Water may also
                    continue to enter the landfill through
                    structural faults. Predictions of rates
                    and amounts of leachate produced can
                    be developed if the hydraulic parame-
                    ters and/or specific-yield values for the
                    hazardous waste and backfill materials
                    are known.
                      A literature search showed that lim-
                    ited hydraulic parameters and specific-
                    yield information  are available. Unit-
                    gradient and specific-yield modeling
                    approaches were evaluated for use at
                    hazardous waste landfills. Specific yield
                    was determined for three data sets:
                    one collected by  a commercial haz-
                    ardous waste landfill operator and pro-
                    vided by the state regulatory agency,
                    one collected by the authors at a haz-
                    ardous waste site located in New York
                    State, and one developed from physical
                    models where drum arrangement, void
                    volume, and soil type were varied.
                      This Project Summary was devel-
                    oped by EPA's Hazardous Waste Engi-
                    neering  Research  Laboratory, Cincin-
                    nati, OH, to announce key findings of
                    the research project that is fully docu-
                    mented in a separate report of the same
                    title (see Project Report ordering infor-
                    mation at back).
Introduction
  Leachate levels rise in closed haz-
ardous waste landfills as waste materi-
als dewater and as precipitation trapped
during  construction and operation
drains under gravitational influences.
This leachate collects on top of the liner
and is pumped or drained by gravity via
leachate collection systems. Under reg-
ulations developed pursuant to the Re-
source Conservation and  Recovery Act
(RCRA), the depth of leachate in the cell
above the liner should not exceed one
foot (approximately 30 cm). Should the
cover or liner not be functioning prop-
erly, additional fluid may enter the land-
fill. When leachate in the cell lies above
the 30-cm standing level, operators will
be required to  pump or drain the cell
until the guideline level has  been
achieved. If models can be developed to
estimate the fluid drainage rate from a
properly functioning closed landfill, de-
viations from the predicted leachate
drainage rate may be used as indicators
of cover or liner performance.
  Previous work on leachate production
from municipal solid waste landfills has
emphasized the concept of field capac-
ity to determine the amount of leachate
produced. Field capacity  is defined as
the amount of water retained by a
porous  material after gravity drainage
ceases and downward water movement
becomes negligible; therefore, in
closed landfills where the waste is satu-
rated, field capacity refers to the  mois-
ture  content after the  landfill has
drained. Hence, the  maximum volume

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of fluid that may drain from  a closed
landfill is the  difference  between the
saturated porosity and the field capacity
and is  denoted as the specific yield or
drainable porosity. The specific yield,
therefore, is essential in determining
the total amount of leachate that may be
drained from a landfill.

Procedures
  This  project was divided into three
main tasks: 1) assess the availability of
data concerning  the  hydraulic proper-
ties of waste and backfill material, 2) re-
view the conceptual models applicable
to investigating the drainage character-
istics of wastes and soil in a landfill en-
vironment, and 3) select and test an ap-
propriate conceptual model to estimate
leachate  production from typical haz-
ardous waste landfills.

Waste Properties
  Information on hydraulic properties
of wastes and soil drainage were found
in the  literature from a wide  range of
disciplines (e.g., chemical, civil, agricul-
tural, and geotechnical engineering; bi-
ology;  geology;  soil  physics; and soil
chemistry). A comprehensive  literature
review was made using computer-
assisted data bases. The data  bases in-
cluded NTIS (for government research
reports), COMPENDEX (for engineering
documents), and BIOSIS (for  biological
and environmental publications).  Key
words, such as landfill, drainage, land-
fill cells, liner, leachate,  conductivity,
and permeability were used to identify
titles and abstracts applicable to  this
study.  In addition, owners and opera-
tors of hazardous waste landfills were
contacted to determine their disposal
techniques and types of  waste forms
handled at the facility. Results of this
literature review are summarized in
Table 1.

Conceptual Models
  Simplifying  the assumptions about
the complexity of leachate flow through
the wastes was  necessary in order to
model  the behavior of hazardous waste
landfills. The two  major  assumptions
made for this analysis were  that 1) all
liquid  flow is vertical and 2) the hy-
draulic properties of  the waste materi-
als are uniform throughout the landfill.
  The  first major assumption that re-
duces the flow geometry from three di-
mensions to  a one-dimensional (1-D)
analysis is dictated  by the  increased
complexity and computational costs as-
sociated with 2-D and 3-D flow models.
Although landfills are 3-D in nature, the
assumption of flow only in the vertical
direction may be valid for  landfills of
regular geometry receiving uniform
area!  recharge. The  assumption  may
not be valid in landfills where surface
soils (covers or daily backfill) or surface
slopes result in increase of runoff in cer-
tain areas of the landfill and  ponding of
precipitation in others. In addition, hori-
zontal hydraulic gradients at the landfill
sidewalls or the presence of a  shallow
water table may produce  significant
components of horizontal flow.
  The second major assumption of uni-
form hydraulic properties ignores the
effects  of the heterogeneity of the
waste. Typical hazardous waste landfills
in the United States handle a wide vari-
ety of solid wastes ranging from con-
taminated soils  and  sludges  in bulk
form to drummed waste and polychlori-
nated biphenol-contaminated trans-
formers. In addition, soil material  is
often placed  between the layers of
drums to allow vehicle movement
within the landfill cell during operation.
Modes  of deposition of the  wastes
range from neat upright  or horizontal
placement of drums  and careful map-
ping of their location to haphazard dis-
posal of drums and containers by
dragline or overhead crane. Bulky waste
material is often dumped and spread by
bulldozer or front-end loader.
  In performing this  project, the focus
was not on a  complete  description of
fluid movement throughout the cell, but
on that amount of fluid draining to the
leachate collection system. The effect of
the leachate collection system is to av-
erage the local drainage  from various
portions of the cell. In similar problems
of agricultural  drainage,  simplified
models of flow mechanisms can be em-
ployed because the details  are lost  in
the process of averaging  over large
areas. As a result of a  review of the
available research on flow in hazardous
waste cells, two types of modeling ap-
proaches were chosen for application:
unit gradient and specific yield.
  The unit gradient approach is applica-
ble to soil or bulk waste cells (i.e., those
cells receiving a uniform, soil-like waste
material). Such wastes might be found
at private  generator/disposer sites
where cells  are used  strictly for a
specific kind  of waste.  These waste
types would be expected to  behave like
soils, and drainage from these materials
has been analyzed as such.
  The second approach  is  for landfill
cells that may not contain  primarily soil-
like material. Currently, many commer-^
cial waste sites dispose of drummed,^
solidified waste. Leachate will be stored
in the large voids between the drums as
well as in the pore spaces of any back-
filled soil. In most cases, the large voids
will contain the most leachate. Under
drainage conditions, these large voids
will easily dewater, leaving the leachate
in the back-filled soil behind. The con-
cept of  free-draining pore  spaces
(voids)  can be used to model fluid
drainage in drum disposal cells.
  Using the assumption that the porous
material  has sufficient time to  drain to
near equilibrium, the specific  yield is
given by the following equation:
          Sy = ^(100)
(1)
where Sy is the specific yield, Q (m3) is
the amount of leachate pumped out of
the landfill, and LFV (m3) is the volume
of landfill drained.
  The models evaluated represent sim-
plified conceptual models of hazardous
waste landfills. Under actual field condi-
tions, no one model may be completely
applicable.  The models chosen are de-
signed for use under a variety of landfill
configurations and waste types. Table 2
(see page 6) outlines the key points of the
models and the typical data needed for
their application.

Selected  Methodology
  The models described were incorpo-
rated into  a  methodology or decision
tree analysis that may be  used to esti-
mate the leachate production from  a
closed landfill. Figure 1 shows the major
components of the decision tree analy-
sis. This analysis is designed to be ap-
plicable to a wide variety of closed land-
fill cells. The methodology begins with
the  first question of: What type of
wastes are  in the landfill? Based on dis-
cussions with operators and regulators,
landfills were found to be composed of
1) primarily bulky, soil-like waste (e.g.,
fly ash,  contaminated soil, metal hy-
droxide sludges), 2) drummed or con-
tainerized waste with small amounts of
backfilled soil, and 3) mixtures of bulky
and drummed waste. All subsequent
analysis in the  methodology requires
that this information be known. The sec-
ond question is How much leachate
(saturated thickness) lies above the
landfill liner? RCRA guidelines state that
the  level of  leachate  shall not exceed
1 foot (approximately 30 cm) above the
liner. Once these questions have been

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Table 1. Reported Hydraulic and Geotechnical Properties of Hazardous Wastes
Saturated parameters
Waste type
Municipal waste
Municipal waste
Papermill sludge
Flue Gas
Desulfurization (FGD)
sludge
Solvay soda ash
sludge
FGD sludge
Municipal waste
FGD sludge
Lead/zinc mill
tailings
Uranium mill tailings
Municipal waste
Uranium mill tailings
Coarse
Medium
Fine
Fly ash
Municipal waste
Municipal waste
Spent oil shale
Municipal incinerator
residue
Phosphate tailings
Coal mine wastes
Waste clays from
phosphate mining
Fly ash/soil mixture
Red mud f aluminum
tailings)
Compacted partially saturated parameters Gr?in
KCAT density s/z£
(cm/s) Porosity (kg/m3) K($) $(H) 6f?£S Field Capacity data
50% 45%
340.0 10-14%
1.0 x 70~4
7.0 x 70-8
7.0 x 70-*
7.0 x 70~6
2.77* X
46-57% 1040-1280 X
20-35%
1.0 x 70~2 2.72-2.52*
7.0 x 70-5
3.4 x 70~6 47-57% 2.88-3.02* X
5.0 x 70~s
2.2 x 70~4 44% 7.63**
X X 30-40%
6.3 x 70-3 43.3% 7.48-7.57** X X 7.6% X
2.3 x 70-3 45.8% 7.28-7.50** X X 9.0% X
6.7X70-7 66.0% 0.90-7.70** X X 31.0% X
8.3 x 70-3 0.99-7.50** X X
5.0 x 10~2 X X
Bulk density
versus field ca-
pacity data
287.0 28.6%

4. 1-6.9 x 70~s
7.2 x 10~4
2.0 x 70-5
7.4 x 70-3
7.22 x 70~5
7.0 x 70~4
7.0 x 70-*
7.0 x 70-3
7.5 x 70-6
7.2-20 x 70-7
Consolidation parameters
Shear
Cv % Vol strength
Icm2/s) E(cr) reduction data



X X
2.87* X X
X X
X
0.75-7.05* X







X X



1.0-6.0 x 70~4

0.09-6.9 x 1Q-2
*—particle density
*—compression index
**—bulk density
X—data not readily available

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Well Drained or Drummed Waste
\<30cm
No Further \ ^ Leachate
Drainage \~ V^. Level
Input
1) Specific Yield
Calibrate
Input
Parameters
Apply Specific
Yield Model
Does
Prediction
Match Field
Data
\
Output
1) Leachate Level
vs. Time
2) Leachate Flux
M -Saturated Hydraulic
Conductivity of Waste
mn =Maximum Allowable
Leachate
<30 cm
Bulk Waste
> 30cm
V^ Level .S

Input
Volume of Backfill
So/7 or Bulk Waste
\
Input
Initial Water Content
Profile of Backfill
Soil or Bulk Waste
t
Input
Waste Hydraulic
Properties

Apply Unit
Gradient Model
JL

Calibrate
Input
Data
t

Input
Volume of Backfilled
Soil or Bulk Waste
i

Input
Waste Hydraulic
Properties
S^is K..t >*
^S^,^^
-------
30 cm
Input
Initial Water Content
of Waste
Input
Waste Hydraulic
Properties
Input
Soil Hydraulic
Properties
Calibrate
Input
Data
Calibrate
Input
Data
Apply Unit
Gradient Model
Apply Unit
Gradient Model
Does
Prediction
Match Field
Data
Does
Prediction
Match Field
Data
Goto
Well-Drained
Methodology
\
Output
1) Leachate Production
vs. Time
\
\
Output
1) Leachate Level
vs. Time
21 Leachate Flux vs. Timt
i/ne\
answered, data must be assembled on
the hydraulic properties of the waste.
The properties needed for the analysis
depend on the answers to the first two
questions.
After the landfill parameters have
been identified, the user must decide
whether the initial flow rates predicted
by either model will exceed the capacity
of the leachate collection and treatment
system. This decision is based on a
comparison of the leachate flux (satu-
rated hydraulic conductivity • head •
area) of the waste to the designed
leachate collection/treatment capacity.
If the leachate flux of the landfill ex-
ceeds the capacity of the system, then
the leachate production rate will be lim-
ited by the system capacity. If this
product is less than the system capacity,
the production rate will be controlled by
the hydraulic properties of the waste. At
this point in the decision tree, the
proper model is applied to the site data,
and leachate production is predicted. If
field data on leachate production are
available, these are compared to the
predicted results and the model may be
calibrated to better predict long-term
leachate production.

Discussion
In this study, specific yield was deter-
mined for three data sets: one collected
by a commercial hazardous waste land-
fill operator and provided by the state
regulatory agency; one collected by the
authors at the Glen Falls, New York site;
and one developed from physical mod-
els where drum arrangement, void vol-
ume, and soil type were varied.

Commercial Site
The commercial site cells are exca-
vated in native clay and lined with both
compacted clay and flexible membrane
liners. Data collected for one of the cells
at the site were analyzed. The cell is di-
vided into five subcells; each subcell is
hydraulically separated using clay
berms to allow for the segregation of
specific materials.
The data collected by the site opera-
tors consisted of daily leachate level
measurements in standpipes for each
subcell and monthly total leachate vol-
umes pumped from each cell. Two
leachate levels were recorded for each
day for each subcell; the level before
pumping and the level measured imme-
diately after pumping stopped. Leach-
ate levels were measured in standpipes
using a weighted string, and pumping
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Table 2. Summary of Conceptual Models

Model Application
Resulting Data
Unit Gradient
a) deep water table
b) shallow water table
Specific Yield
Bulk waste forms, known
hydraulic properties, uni-
form initial water content
profile, deep landfill

Same as above except
shallow landfill
Unknown hydraulic proper-
ties, drummed or rapidly
draining waste forms
Drainage flux versus
time, water content
profile versus time
Drainage flux versus
time, water content
profile versus time

Leachate level versus
time
volumes were measured using totaliz-
ing flow meters.
Specific yield was calculated over two
time periods. The first time period was
from September 1982 to September
1983, the second from October 1983 to
January 1984. The first of these periods
reflects the time during which a tempo-
rary cover was in place. The second pe-
riod is after final cover placement.
Cover integrity and collection system
efficiency can be determined by exam-
ining data from these time periods.
Using Equation 1, the results of these
analyses indicate that the specific yield
was 6.6% during the first time period
and 11.0% during the second time pe-
riod. The increase in specific yield (after
the final cover was installed) tends to
indicate that the temporary cover was
as effective in reducing infiltration as
the final cover.

Glen Falls Site
An existing hazardous waste site in
New York State was selected for instal-
lation of an automated water level
recorder to help determine a typical
specific yield value for mixed waste.
The automated water level recorder
was used to determine the changes in
water level associated with leachate re-
moval. This waste site had an ineffec-
tive clay cap, which was scheduled for
replacement during the summer of
1985. The liner was constructed from
two layers of clay separated by gravel
with a leak detection system located in
the gravel. No detectable leakage from
the waste site had occurred. At the end
of August 1984, a leachate level of 3.6 m
was observed above the top clay liner.
The leachate level was monitored as
leachate was removed from the waste
site in 25.0 m3 increments.
The calculated specific yield for the
Glen Falls Site was 18.0%. However, cal-
culation of the precipitation entering the
wastes through the cover indicates that
as much as 52% of the leachate pumped
may have been contributed by sources
outside of the landfill. This being the
case, the specific yield would decrease
to 8.0% for the Glen Falls Site.

Physical Models
Physical models were constructed to
represent various configurations of haz-
ardous waste and backfill materials.
Drum arrangement, void volume, and
soil type were evaluated by simulating a
section of landfill containing 8 to 10
drums, each 0.21 m3 in volume, and
backfill soil material. For each physical
model constructed, the drum volumes
and large interdrum void spaces were
calculated. Drainage was measured by
continuously weighing the entire
model. Drainage rates from the physical
model were restricted by the outflow
pipe conductivity and the soil column
flow resistance. Drainage rates at any
time were determined as a function of
the outflow pipe resistance, treatment-
dependent soil resistance, and hy-
draulic head of the saturated soil
column in the model.
The specific yield for the entire tank
measured for each treatment is depend-
ent on the presence of large void space,
soil volume available for drainage, soil
drainage characteristics, and the
amount of soil column discontinuties
introduced by large void spaces. Values
are presented in Table 3 for total
drainage, drum volume, void volume,
specific yield, maximum drainage rate,
and time required for the drainage rate
to decrease below an arbitrarily chosen
flux of 5.46 x 10~4 cm/s for each treat-
ment. Calculation of specific yield for all
treatments was based on Equation 1
where Q is the average leachate volume
drained from that treatment and LFV is
the filled volume of the tank.

Conclusions and
Recommendations
Based on the results of the research, it
is apparent that numerical and analyti-
cal models are either too complex or
require characterization data that are
not currently available. Even the appli-
cation of the specific yield model re-
quires generally unavailable informa-
tion about the volumes of drainable
large voids, specific yield or water re-
tention, and hydraulic conductivity val-
ues for the backfill material and the vol-
ume of nondraining solids at hazardous
waste landfills.
Very few values of specific yield have
been reported for either nonhazardous
or hazardous waste sites. Estimates of
specific yield can be determined from
single withdrawals of leachate from a
waste site if care is taken to account for
possible infiltration of precipitation or
changes in barometric pressure during
measurements of leachate levels. It is
highly recommended that leachate lev-
els be monitored with an automatic data
collection system for several days be-
fore and after each leachate withdrawal.
The data collection system should also
record barometric pressure, tempera-
ture, and precipitation data.
The effect of drum arrangement, void
volume, and soil type on specific yield
were examined in a physical model. Val-
ues of specific yield were shown to be
most sensitive to the presence of large
voids. The effects of soil type on
drainage rates and specific yield were
also observed; fine soil retained more
water and had lower drainage rates. The
presence of nondraining solids gener-
ally reduced the drainage rate.
Because useful information is cur-
rently not available for existing haz-
ardous waste sites it is recommended
that a protocol be established that re-
quires careful characterization of waste
sites as they are built. Because site fail-
ure is always a possibility, site charac-
terization information describing all as-
pects of waste form, placement, and
burial should be part of the site history.
Only if landfill design and waste han-
dling information is available, can rea-
sonable predictions of leachate produc-
tion be made without extensive
assumptions or potentially dangerous
site characterization investigations to
estimate the specific yield of the haz-
ardous waste landfill under study. It is
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Table 3. Physical Model Results
Treatment
Time Elapsed
Large Maximum Until Drainage
Average Drum Void Specific Drainage Falls Below
Drainage Volume Volume Yield Rate 5.46 x 10~4 cm/s
(kg) (m3) (m3) Tank (cm/s) (min)
Sand, 987.2 -0- -0- 0.24 4.46 x W~2
No Drums
Sand, 430.2 2.08 -0- 0.11 1.74 x 10~2
Single Layer,
No Voids
Sand, 666.0 1.87 0.35 0.17 2.90 x 10~2
Vertical, Voids
Sand, 536.4 1.87 -0- 0.13 2.06 x W~2
Vertical,
No Voids
Sand, 1130.5 2.08 0.77 0.28 6.52 x W~2
Single Layer,
Voids
Sand, 467.4 1.98 -0- 0. 12 2.38 x 10~2
Double Layer,
No Voids
Loamy Sand, 238.8 2.08 -0- 0.06 8.92 x 10~3
Single Layer,
No Voids
Loamy Sand, 1125.0 2.08 0.90 0.28 4.67 x 10 ~2
Single Layer,
Voids
105

202

recommended that a new research ef-
fort be initiated to define acceptable
construction and as-built reporting
criteria for all new hazardous waste
landfill construction. Once the reporting
criteria are established, procedures for
prediction of leachate production can
be developed for use by landfill opera-
tors and regulatory agencies.
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R. R. KirkhamandG. W. Geeare with the Pacific Northwest Laboratory, Richland.
WA 99352; S. W. Tyler is presently with the Desert Research Institute.
Jonathan G. Herrmann is the EPA Project Officer (see below).
The complete report, entitled "Estimating Leachate Production from Closed
Hazardous Waste Landfills," (Order No. PB 86-207 503/AS; Cost: $11.95,
subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Hazardous Waste Engineering Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
BULK RATE
POSTAGE & FEES PAID
EPA
PERMIT No G-35
Official Business
Penalty for Private Use $300
EPA/600/S2-86/057
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